1. Field of the Invention
The present invention relates to an image forming apparatus, such as a digital copying machine, for forming a color image, and an image forming method.
2. Description of the Related Art
In a conventional image forming apparatus, a cleanerless process, in which a cleaner such as a blade is not provided on a photoconductor body surface, is an advantageous technique for reduction in size of the apparatus or in reduction in amount of toner consumed. There have been various inventions relating to this technique. For instance, U.S. Pat. No. 4,727,395, Jpn. Pat. Appln. KOKAI Publication No. 59-133573 and Japanese Patent No. 2879883 disclose simultaneous development/cleaning techniques in a reverse development process.
This technique is particularly effective even in a full-color process of image forming apparatus that have been developed in recent years. The technique has recently been adopted in 4-series tandem type apparatus.
The cleanerless process has three merits:
1. Since a photoconductor body cleaner is needless, the structure is simplified.
2. Since the photoconductor body is not abraded by a cleaner, the life of the photoconductor body is elongated.
3. Since waste toner is recovered and reused, the toner consumption efficiency increases and no waste toner is produced.
However, the 4-series tandem type color image forming apparatus has the following two demerits, which weaken the advantageous effects:
1. Reverse transfer occurs from a front-stage color station to a rear-stage color station, color mixing occurs depending on the type of an image to be printed, and consequently a color hue may vary.
2. It is necessary to print a pattern, an image, etc. for maintaining image quality, even on a transfer belt or an intermediate transfer medium. As a result, waste toner is produced. Even if a cleaner for a photoconductor body is dispensed with, a waste-tonerless system cannot be realized in the entire apparatus.
The cleanerless process in an ordinary image forming apparatus has the following three demerits:
1. Since the photoconductor body is not abraded by a blade, filming of toner (fixation of toner) may occur on the photoconductor body surface if the photo-conductor body surface has poor compatibility with the toner.
2. Since post-transfer residual toner passes by a charging section and an exposing section, memory may easily occur on the image due to, in particular, the effect relating to the exposing section.
3. There is a concern about color mixing in a color process, as mentioned above.
In recent years, when an attempt to enhance the image quality is made by using a toner with a relatively high sphericity, such as a polymer toner, or a toner with a small grain size, a proper margin becomes narrower with use of a blade cleaner, compared to the case of using conventional toner, and it becomes difficult to obtain an enough life of the cleaner or photoconductor body. From this standpoint, too, attention has recently been paid to the cleanerless process that does not require a blade cleaner. Furthermore, in the cleanerless process, the transfer efficiency is improved by using the above-mentioned toner. Thus, the image quality can be maintained to a certain degree even in the cleanerless process.
Even in this situation, reverse transfer frequently occurs depending on the kind of paper to be used. In particular, when thick paper is used, a sufficient performance is difficult to achieve. Jpn. Pat. Appln. KOKAI Publication No. 2003-162182, for instance, discloses an example in which a special condition is set for a thick-paper mode. In this publicly known example, a blade cleaner is not provided on the photo-conductor body. Using a charger, a control is executed to discharge reverse-transfer toner into a developing device or a cleaner of a transfer belt, etc.
In this method, however, after thick paper is fed, an excess time is needed for the discharge, etc. In addition, a load on the charger itself is large, and consequently the life of the charger may be shortened or the performance of the charger may deteriorate thereafter.
There is a method wherein a dedicated cleaner for the photoconductor body is not used, and the following complex operation or control is performed. That is, toner is once recovered by a charger, etc., and the recovered toner is discharged onto the transfer belt at a non-printing time, and then the toner is recovered by belt cleaner. In this method, however, an exact control of the charge polarity of toner is difficult. This method is disadvantageous in terms of image quality, compared to the case of using a dedicated cleaner.
If no importance is placed on the life or cost, it is better to clean the photoconductor surface once image formation is completed, and thereby a higher image quality is obtained. Besides, it is difficult to achieve a high resolution if optimization for a cleanerless process is executed and an exposure condition is set for making memory less visible. However, as regards whether a high image quality is always necessary for the user at high cost, it depends greatly on the user's sense of value.